The present invention relates to a system for monitoring the condition of a switch such as a pressure-responsive switch in a vehicle tire.
Remote switch monitoring systems for determining a physical condition have been proposed in the prior art. Many of the proposed systems are designed particularly for monitoring pressure in a vehicle tire. Generally, these systems include a transmitter and receiver mounted on a vehicle body and, carried on a wheel in the vehicle, a passive-response switch circuit which responds to either a low or high pressure condition in the tire. Inductive coupling between the vehicle and wheel-mounted circuits produces a receiver signal whose characteristics are indicative of the state of the switch in the switch circuit. The system can therefore be used to inform the vehicle operator of a change in tire pressure, typically the loss of pressure in the tire.
One such prior art device for monitoring vehicle tire pressure is disclosed in U.S. Pat. No. 2,274,557 to Morgan, et al. The device, which incorporates the transmitter, receiver and switch elements of the type mentioned above, employs a pressure responsive variable capacitor to vary resonance characteristics of the switch circuit in response to tire pressure changes. U.S. Pat. No. 4,074,227 to Kalmus describes a similar type of system in which a passive wheel-mounted switch circuit is constructed to respond to a transmitter modulating frequency only at the closed condition of a pressure-response switch in the circuit. A similar type of tire pressure indicator is shown in U.S. Pat. No. 3,723,966 to Mueller, et al.
A variety of coil configurations for producing inductive coupling between the vehicle and wheel mounted circuits have been proposed. U.S. Pat. No. 4,057,783 to Blanchier describes a remote switch-condition sensor containing magnetically coupled transmitter and receiver coils. Each of these coils is adapted to be inductively coupled to one of a pair of inductor coils in a wheel-mounted switch circuit, such that when a pressure-responsive switch in the circuit assumes a closed condition the effective coupling between the transmitter and receiver coils is changed. In U.S. Pat. No. 4,103,283 to Lee, strong coupling between the coils in a vehicle-mounted transceiver unit and a wheel-mounted switch circuit is effected by the use of ferromagnetic-core coil windings. A change in the switch condition in the switch circuit produces a resonance shift in the transceiver unit, to indicate a change in the switch state.
The present invention includes a remote switch monitoring system which provides unique advantages over prior art remote sensing devices. One important advantage provided by the present invention is enhanced signal induction in a receiver unit in the system due to the presence of a novel enhancer circuit in the system. This feature allows sensitive, reliable signal detection without the need for heavy ferromagnetic cores in the system's circuit coils for producing strong coupling between circuits in the system. Another advantage of the present invention is the ability to produce one receiver signal when an open/closed condition switch in the system is in one condition, by exciting the system at one transmitter frequency, and to produce a second receiver signal when the switch is in its other condition, by exciting the system at a second transmitter frequency. This feature potentially allows both conditions of the switch to be actively monitored at all times during the vehicle operation.
It is a general object of the present invention to provide a remote switch monitoring system which provides the advantages just named.
A more specific object of the invention is to provide such a system for use in monitoring vehicle tire pressure.
Yet another object of the invention is to provide such a device which is relatively inexpensive, easily maintained and reliable.
Broadly, the system of the invention includes a transmitter and a receiver mounted on a first body, such as a vehicle, and magnetically coupled inductor and enhancer circuits carried on a second body, such as a wheel of the vehicle. The inductor circuit contains an open/closed condition switch which is responsive to an environmental condition of the second body, such as a pressure-responsive switch responsive to the pressure of a tire carried on the wheel. The transmitter signal, acting through the inductor and enhancer circuits, produces a frequency-dependent response signal in the receiver whose characteristics are indicative of the condition of the switch in the inductor circuit.
In one embodiment of the invention, the inductor and enhancer circuits are tuned to enhance the response voltage in the receiver, at a selected transmitter frequency and inductor circuit switch condition. In a second embodiment of the invention, the inductor and enhancer circuits are tuned to produce one receiver signal-to-noise peak, at one transmitter frequency and with the switch in one condition, and a second receiver signal-to-noise peak, at a second transmitter frequency and with the switch in its other condition.
These and other objects and features of the invention will become more fully apparent when the following detailed description of the invention is read in conjunction with the accompanying drawings.